George Fytas

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Phonons are the quanta of the acoustic field and can be considered as vibrational energy and information carriers. Structured materials that can manipulate the flow of elastic energy are termed phononics, in analogy to the control of light by photonic crystals. Phonon propagation in architected soft materials reveals a wealth of unexpected findings not only near the Brillouin zone (BZ) but also at long wavelengths compared to the structure periodicity.  The development of microfabrication techniques enables control of the flow of acoustic waves in the GHz region that commensurate the light in the visible spectrum through opening of structure-and component related stopbands and direction dependent elasticity triggering applications in optomechanics and telecommunications. The key function in phononics is the dispersion relation, ω (q) (frequency vs wave vector), which can nowadays be recorded by Brillouin light scattering (BLS) and a fraction of ω (q) by pump-probe picosecond acoustic techniques. A selected example of polymer and colloid based hypersonic phononics is one-component hybrid materials exemplified by polymer grafted (inorganic core) nanoparticles (GNP). GNPs combine strong vibration resonant core (size and elasticity) and transformative polymer conformation depending on physical (graft length, grafting density) parameters. The structure of GNP solid sensitively controls the resonance-induced bandgap below BZ (metamaterials). Scrutinizing structure-controlled mechanics and metamaterial behaviour helps establish a reliable predictive power that will open new application pathways of soft-matter-based high-frequency phononics.